In recent years, the absorbent resin powder has come to find utility in various fields specializing in such hygienic articles as sanitary napkins and disposable diapers and water-retaining agents. The absorbent resin of this quality has been generally produced by polymerizing a resinous raw material, drying the resultant polymer, and pulverizing the dry polymer with a pulverizer. The absorbent resin powder resulting from the pulverization, therefore, contains minute particles short of desired particle sizes. When the absorbent resin powder containing the minute particles is put to use, it has the disadvantage that the minute particles are drifted.
Thus, the practice of incorporating a drift-preventing agent in the resin powder for the purpose of improving the dry flowing property of the resin powder as disclosed in the specification of JP-A-52-121,658 and the practice of incorporating a dust-proofing agent in the resin powder for the purpose of curbing the drifting of minute particles contained in the resin powder as taught in the specification of JP-A-63-39,934 have been heretofore followed.
Further, the method for effecting removal of the minute particles from the resin powder by the use of a sieve and the method for enlarging only the minute particles in the resin powder by the use of a binder have been conceived. The former method, however, is undesirable on account of poor economy. The latter method generally requires use of an organic solvent type binder and, therefore, not only entails the danger of ignition during the step of drying subsequent to the size enlargement but also jeopardizes the biological safety owing to the residue of the organic solvent. The latter method is liberated from this problem when the binder to be used therein is in the form of an aqueous solution. Since the absorbent resin powder produced by pulverization with a binder has the nature of quickly absorbing the aqueous solution, however, the aqueous solution encounters difficulty in allowing uniform dispersion and mixture of the minute particles therein and the agglomeration tends to form large lumps of high density. When the large lumps are to be pulverized, this pulverization entails occurrence of minute particles and produces uniformly aggregated particles only with difficulty.
For the solution of this problem, the method which as taught in JP-A-61-97,333 and JP-A-61-101,536 comprises uniformly mixing the absorbent resin powder and an aqueous liquid and agglomerating the resultant mixture by the use of a specific mixing device such as, for example, a high-speed rotary paddle type mixer or an air current type mixer and then crushing the produced agglomerate has been heretofore tried.
It has been found, however, that the method which effects the mixture of the absorbent resin powder with the aqueous liquid by virtue of the shearing force of the high-speed rotary type mixer as described above produces a powder of an unduly small basic particle diameter. This adverse phenomenon may be logically explained by a supposition that when the resin powder is mixed by stirring in the mixer, the component particles of this powder undergo repetitious mutual collision and, at the same time, yield to fracture due to mechanical shear. When the particles of the resin powder having the particle surface thereof specially treated in advance for the purpose of qualitative improvement are fractured as described above, the fractured particle surface poses a problem of qualitative degradation.
When the air current type mixer mentioned above is to be adopted, it has a problem of unfitness for commercial production of an absorbent resin powder because it is incapable of continuous stirring. Moreover, the mixer has another problem of practically uneasy maintenance because the otherwise inevitable adhesion of particles of the powder to the mixer must be prevented by keeping the resin powder heated and meantime adding thereto the aqueous liquid piecemeal for a long time.
JP-A-1-236,932 proposes a spray agglomerating apparatus for effecting continuous agglomerating of a powder. This spray agglomeration apparatus comprises a drying chamber, a powder supply device provided with a powder discharge outlet disposed above the drying chamber, and spray nozzles disposed one each on the opposite sides of the powder discharge outlet and adapted to spout mutually intersecting currents of liquid droplets.
This apparatus is effective for the purpose of effecting continuous agglomeration of the powder of dextrin or melamine resin, for example. When this apparatus is adopted for the agglomeration of an absorbent resin powder with an aqueous liquid, however, it fails to accomplish the primary object of continuous size enlargement because viscous agglomerate of the absorbent resin which have acquired increased viscousness due to absorption of water adhering to the wall of the apparatus and the deposit of viscous agglomerate thus formed gains in volume with the elapse of time. Further, when the absorbent resin powder and the aqueous liquid contact each other at the position of mutual intersection of the projected threads of the aqueous liquid, the dispersion of the mixture caused by the force of collision due to the discharge of liquid droplets is insufficient and the state of contact is ununiform. The enlarged particles consequently obtained, therefore contain minute particles which have escaped the size enlargement and suffer occurrence of wet clusters which have absorbed water in a large amount.
In view of the true state of prior art described above, we made a diligent study in search of a method for agglomeration which comprises subjecting an absorbent resin powder to continuous agglomeration thereby attaining elimination of minute particles contained in the resin powder and an apparatus for execution of the method. We consequently acquired a knowledge that from an absorbent resin powder containing minute particles, an absorbent resin powder of high quality possessing particle sizes in a desired range is obtained by causing the resin powder and an aqueous liquid separately dispersed in advance with the current of air to flow down a cylindrical member from the upper part to the lower part thereof and come into parallel flow contact during the descent thereby inducing cohesive union of the absorbent resin powder containing minute particles and optionally crushing coarse clusters possibly formed in consequence of the union. This invention has been perfected as a result.
An object of this invention, therefore, is to provide a method for continuous agglomeration which accomplishes elimination of minute particles contained in an absorbent resin powder by agglomerating the individual particles of the absorbent resin powder by cohesive union with an aqueous liquid as a binder while avoiding mutual collision thereof and optionally crushing coarse particles possibly formed by the cohesive union and an apparatus for the execusion of this method.